Combine rhubarb, cherries, sugar, and tapioca; let stand 15 minutes.,Combine flour and salt in bowl; cut in shortening with pastry blender until mixture resembles cornmeal.,Sprinkle water over mixture; stir ingredients quickly with fork until blended; gather into a ball; divide in half.,Roll out one half to a 12-inch circle; fit into 9-inch pie plate. Trim even with edge of pan; moisten edge with cold water.,Spoon filling into pastry; dot with butter.,Roll out second half of pastry as the first; fold in half; cut designs or slashes in top to vent steam. Place over filling with fold at center; unfold. Trim upper crust 1/2 inch beyond edge of plate. Fold edge of top crust under edge of bottom crust. Flute with fingers.,______